1. Field of the Invention
The present invention relates to a blood pump used for heart-lung machines or the like.
2. Prior Arts
In recent years, centrifugal blood pumps have begun to be used widely as blood circulation pumps for heart-lung machines.
In the case of the centrifugal blood pump described in U.S. Pat. No. 4,507,048, a nearly conical impeller with side vanes equipped with a magnetic means, such as permanent magnets, on the bottom surface side thereof is supported at pivots located at the upper and lower ends of the rotation shaft thereof by the pivot bearings of the casing which encases the impeller and is provided with a suction inlet and a delivery outlet. A magnetic drive means, such as an electric winding for generating a rotating magnetic field, being disposed outside the casing opposite to the above-mentioned magnetic means with the casing interposed therebetween in order to rotate the impeller in cooperation with the magnetic means.
In this kind of blood pump, the impeller is attracted to the bottom surface of the casing by the magnetic attraction force generated between the magnetic means disposed on the bottom side of the impeller and the magnetic drive means disposed opposite to the magnetic means with the casing interposed therebetween. When the rotation speed of the impeller is low, the impeller rotates mainly supported by the pivot at the lower end of the rotation shaft thereof and the bearing for the pivot. When the rotation speed of the impeller increases, the blood pressure in the casing becomes low at the upper side of the impeller and becomes high at the lower side thereof, thereby generating lifting force to the impeller. When this lifting force exceeds the magnetic attraction force, the impeller becomes supported mainly by the pivot disposed at the upper end of the rotation shaft and the bearing for the pivot. When the magnetic attraction force is balanced with the lifting force, the impeller is in a weightless condition, and the impeller performs ideal rotation wherein no great force is applied to the pivot bearing.
In the above-mentioned conventional blood pump, however, since the gap between the magnetic means disposed on the impeller and the magnetic drive means disposed outside the casing opposite to the magnetic means is set to a fixed value, the magnetic attraction force generated therebetween is constant. However, the above-mentioned impeller lifting force is changed by pump operation conditions, such as impeller rotation speed and blood flow rate. For this reason, it is difficult to balance the magnetic attraction force with the lifting force. In addition, the rotation of the impeller becomes unstable depending on the operation conditions of the pump, causing a problem of generating vibration on the impeller. In particular, when the rotation shaft of the impeller is supported by the pivot and the bearing for the pivot, the curvature radius R of the bearing support surface is usually made far larger than the curvature radius r of the pivot tip as shown in FIG. 2. Since the pivot is not supported in the radial direction thereof, the pivot is apt to generate vibratory rotation, thereby significantly causing the above-mentioned problem.
When the impeller vibrates, blood cells in blood are destroyed and more hemolysis is caused. Furthermore, the pivot and the bearing for the pivot are worn greatly, thereby causing a problem of making continuous operation for an extended period of time difficult.
Moreover, if axial dislocation or inclination is generated between the rotation center axis of the magnetic means of the impeller and the center axis of the magnetic drive means disposed outside the casing, opposite to the magnetic means, the magnetic attraction force generated during impeller rotation does not become uniform, thereby making impeller rotation unstable and causing problems similar to those described above.
In U.S. Pat. No. 5,399,074, the applicants of the present invention have proposed that the pivot or the bearing for the pivot at the rotation shaft lower end of the impeller of the blood pump should be made of ceramics. The specification of the Unites States Patent has also disclosed that at least the pivot or the bearing of each of the pivot-bearing combinations is desired to be made of ceramics (in line 39 of column 6 and the following descriptions). However, if both the pivot at the rotation shaft end of the impeller and the bearing for supporting the pivot are made of ceramics, these members are worn significantly during impeller rotation. As a result, impeller rotation becomes unstable during operation for an extended period of time, thereby causing problems similar to those described above.